1. Field of the Invention
This invention relates to the field of wide screen television receivers and video displays, and in particular, to wide screen television receivers and video displays in which different display modes having different picture sizes are implemented by manipulating deflection circuits and raster sizes.
2. Description of Related Art
When referring to a television picture, there are actually two aspect ratios. One aspect ratio is that which describes the outer boundary or borders of the picture. In conventional television receivers, this aspect ratio is 4:3. The horizontal factor usually appears first. In wide screen television receivers, this aspect ratio is, for most manufacturers, 16:9. The second aspect ratio is that which describes the images which are displayed on the television screen. A perfectly round circle, for example, which appears on a television screen as a perfectly round circle, has an aspect ratio of 1:1. If the same circle is displayed on a wide screen television receiver, without any corrective measures, the circle will appear to be an oval, which is elongated along the horizontal axis. If the receiver is 16:9, the circle will have an aspect ratio of approximately 1.3:1. If the same circle is displayed on a wide screen television receiver, with appropriate corrective measures, the circle will appear to be a perfect circle, and will have an aspect ratio of 1:1. Two expressions are used herein regarding aspect ratio in order to avoid confusion. One expression is "format display ratio" and the other is "image aspect ratio".
The expression "format display ratio" refers to the width to height ratio of the boundaries or borders of a picture. The format display ratio of a video signal from a conventional video source is 4:3. The format display ratio of a conventional television receiver is 4:3. The format display ratio of a video signal from a wide screen video source is 16:9 (sometimes 5:3, sometimes another wide ratio). The format display ratio of a wide screen television receiver is 16:9 (sometimes 5:3, sometimes another wide ratio).
The expression "image aspect ratio" refers to the width to height ratio of images within a picture. Images having an image aspect ratio of 1:1 will appear without image aspect ratio distortion. Images not having an image aspect ratio of 1:1 will appear with image aspect ratio distortion.
Wide screen television receivers and displays have several particularly useful, and in some cases critical, display modes. These display modes include: displaying pictures from video sources having conventional format display ratios, without image aspect ratio distortion; displaying pictures from video sources having wide screen format display ratios, without image aspect ratio distortion; displaying pictures from video sources having conventional format display ratios, in an enlarged size, without image aspect ratio distortion.
Displaying pictures from video sources having conventional format display ratios, in an enlarged size and without image aspect ratio distortion, often results in a portion of the picture at the top and bottom being cropped. This portion is one-third, if the wide screen display is 16:9 and the size is enlarged horizontally and vertically by factors of four-thirds. If the enlarged picture is vertically centered, then one-sixth of the picture will be cropped from the top and one-sixth of the picture will be cropped from the bottom.
This cropping raises two operating mode issues. One issue is that a viewer might be willing to accept a certain amount of image aspect ratio distortion in order to reduce cropping and see more of the picture. In this case, another desirable display mode would be displaying pictures from video sources having conventional format display ratios, in an enlarged size, with a small but acceptable amount of image aspect ratio distortion. The other issue is that subtitles and other kinds of information might be in the cropped part of the picture. A viewer might very well consider the subtitles or other information to be critical, but is unwilling to accept image aspect ratio distortion. In this situation, yet another desirable display mode would be displaying pictures from video sources having conventional format display ratios, in an enlarged size and without image aspect ratio distortion, but panned upwardly so as to display the subtitles or other information and crop only, or substantially only from the top of the picture.
A vertical shift is required to make the hidden portions of the picture visible. This can be obtained by superimposing a DC component on the vertical deflection current. This requires a vertical deflection amplifier with DC coupling and sufficient output current range. Such an amplifier has the disadvantage of increased dissipation losses. Another approach is to use a floating DC current source connected in parallel to the vertical deflection yoke. The dynamic range of the output voltage of the current source must be large enough to account for the flyback pulse. A disadvantage of current superposition is that the vertical linearity correction and the East-West side pincushion distortion correction have to be readjusted when the vertical shift is changed.
U.S. Pat. No. 5,287,042 discloses a switching arrangement to control the horizontal deflection amplitude. In a first condition, a raster is generated having a deflection amplitude and a deflection correction parameter corresponding to a first aspect ratio. In a second selectable condition, a raster is produced having a deflection amplitude and deflection correction parameter corresponding to a second aspect ratio. An additional inductance is coupled to the deflection yoke and is short-circuited in one selectable condition in order to conduct deflection current via the inductor.
EP A 0578162 discloses a vertical panning system including a panning circuit which generates a vertical reset signal which is phase shifted by a selected number of horizontal lines relative to the vertical synchronizing component of the video signal.
Implementing all of the display modes described above can be expensive and require considerable signal processing power. These display modes also create problems in correcting typical raster distortion problems, such as side pincushion distortion, as the picture or raster size is changed.